Class 8 Science

Chapter 1 Crop Production and Management

Exercise Questions

1. Select the correct word from the following list and fill in the blanks.

Soln:

(a) The same kind of plants grown and cultivated on a large scale at a place is called
the crop.

(b) The first step before growing crops is the preparation of the soil.

(c) Damaged seeds would float on top of the water.

(d) For growing a crop, sufficient sunlight and water and nutrients from the soil are
essential.

2. Match items in column A with those in column B.

A B

(i) Kharif crops (a) Food for cattle

(ii) Rabi crops (b) Urea and superphosphate

(iii) Chemical fertilisers (c) Animal excreta, cow dung urine and plant wastes

(iv) Organic manure (d) Wheat, gram, pea

(e) Paddy and maize

Soln:

(i) Kharif crops (e) Paddy and maize

(ii) Rabi crops (d) Wheat, gram, pea

 (iii) Chemical fertilisers (b) Urea and superphosphate

(iv) Organic manure (c) Animal excreta, cow dung urine and plant wastes

3. Give two examples of each.

(a) Kharif crop

(b) Rabi crop

Soln:

Kharif crops – Paddy and maize

Rabi crops – Wheat and pea

4. Write a paragraph in your own words on each of the following.

(a) Preparation of soil

(b) Sowing

(c) Weeding

(d) Threshing

Soln:

a) Preparation of the soil is the first step in agriculture. Preparation of the soil is done to
loosen the soil, which is essential for root penetration into the soil. This allows the roots
to breathe easily. Loosening of the soil allows the growth of earthworms and
microorganisms, which will help to keep the soil fertile by adding humus to the soil.
Loosening of soil also brings nutrient-rich soil to the top layer, which is essential for the
growth of plants.

b) Sowing is an important process in crop production. First, healthy seeds are selected
before sowing. After selecting healthy seeds, sowing is done by either traditional
methods or by using the equipment’s seed drill.

c) Removal of unnecessary plants from the field is called weeding. Weeds compete
with crop plants for nutrients and water. This will reduce the yield of the desired crops.
Weeds also interfere while harvesting and get mixed with crops. Some weeds are
poisonous to animals and humans. Tilling is a common method that helps remove
weeds before sowing crops, and manual methods like the physical removal of plants
are also used to remove weeds. Weedicides are sprayed to get rid of weeds, but this
method may affect the health of farmers because of the chemicals used as
weedicides.

d) Process of separating the chaff from the crop is known as threshing. Threshing is
carried by a machine called ‘combine’, which is a harvester as well as a thresher.
Threshing is also done by winnowing, where the blow of air is used to separate the
chaff from crops.

5. Explain how fertilisers are different from manure.

Soln:

Fertilisers Manure

Fertiliser is an inorganic substance. Manure is an organic substance.

Fertilisers are prepared artificially. Manure is obtained by the decomposition of animal, plant
and human waste.

Prepared in factories. Prepared in fields.

Does not provide humus to the Provide humus to the soil.

soil.

Rich in plant nutrients. Relatively less plant nutrients.

Long-term usage has adverse Long-term usage improves soil fertility.

effects on soil.

6. What is irrigation? Describe two methods of irrigation which conserve water.

Soln: The supply of water to crops at regular intervals is called irrigation. Methods of
irrigation which conserve water are as follows:

a) Drip Irrigation: Here, the water goes drop by drop directly into the roots; this method
is very useful as it conserves the water and also helps in avoiding weeds.

b) Sprinkler system: This method is in use mainly in uneven land where sufficient water
is not available. The perpendicular pipes, having rotating nozzles on top, are joined to
the main pipeline at regular intervals. When water is allowed to flow through the main
pipe under pressure with the help of a pump, it escapes from the rotating nozzles. It
gets sprinkled on the crop as if it is raining.

7. If wheat is sown in the Kharif season, what would happen? Discuss.

Soln: Wheat crops may get destroyed if sown in the Kharif season because of
unfavourable temperatures, pests and adaptable conditions for the plants to grow.
Kharif comes during the rainy season; hence it is not a wise idea to grow wheat in the
Kharif season.

8. Explain how soil gets affected by the continuous plantation of crops in a field.

Soln: Plants require nutrients for their growth. Without optimum nutrients, plants will die.
Continuous plantation of crops results in the depletion of certain nutrients like Nitrogen,
Phosphorus, Potassium, etc. This results in a decrease in yield due to loss of nutrients;
hence there should be a gap between crops in order to get a good yield.

9. What are weeds? How can we control them?

Soln: In a field, many other undesirable plants may grow naturally along with the crop.
These undesirable plants are called weeds. Weeds can be controlled by methods
called weeding. Tilling before sowing the crops helps in uprooting and killing weeds,
which may then dry up and get mixed with the soil. Weeds are also controlled by using
certain chemicals called weedicides.

10. Arrange the following boxes in proper order to make a flow chart of sugarcane
crop production.
11. Complete the following word puzzle with the help of the clues given below.

Down

1. Providing water to the crops.

2. Keeping crop grains for a long time under proper conditions.

5. Certain plants of the same kind are grown on a large scale.

Across

3. A machine used for cutting the matured crop.

4. A rabi crop, which is also one of the pulses.

6. A process of separating the grain from the chaff.

Chapter 3 Synthetic Fibres and Plastics
Exercise Questions

1. Explain why some fibres are called synthetic.

Soln:

Some fibres are called synthetic fibres because they are man-made fibres prepared
using chemicals. These are made of small units that join together to form long chains.
Some examples of synthetic fibres are nylon, rayon, acrylic, polyester etc.

2. Mark the correct answer.

Rayon is different from synthetic fibres because

(a) it has a silk-like appearance.

(b) it is obtained from wood pulp.

(c) its fibres can also be woven like those of natural fibres.

Soln: The answer is (b) it is obtained from wood pulp.

3. Fill in the blanks with appropriate words.

(a) Synthetic fibres are also called __________ or __________ fibres.

(b) Synthetic fibres are synthesised from the raw material called __________.

(c) Like synthetic fibres, plastic is also a __________

Soln:

(a) Synthetic fibres are also called artificial or man-made fibres.

(b) Synthetic fibres are synthesised from a raw material called petrochemicals.

(c) Like synthetic fibres, plastic is also a polymer.

4. Give examples which indicate that nylon fibres are very strong.

Soln:

Following are the examples that indicate nylon fibres are very strong:

• Parachutes and ropes of rock climbing are made of nylon

• Nylon is used in making seat belts, fishing nets and tyre cords.
• Nylon is used in making sports accessories like rackets.
5. Explain why plastic containers are favoured for storing food.

Soln:

Plastic containers are favoured for storing food because plastic is non-reactive and will
not react with the ingredients of the food items.

6. Explain the difference between thermoplastic and thermosetting plastics.

Soln:

The difference between thermosetting plastics and thermoplastics are as follows:

Thermosetting plastics Thermoplastics

It cannot be bent; it will break if we attempt to Thermoplastic can be bent easily

bend thermosetting plastics

On heating thermosetting plastics, they cannot be On heating the thermoplastics, it becomes

softened. This is the reason it cannot be reshaped softened and can be moulded and
once it is moulded. reshaped easily.

7. Explain why the following are made of thermosetting plastics.

(a) Saucepan handles

(b) Electric plugs/switches/plug boards

Soln:

a) Thermosetting plastics are used to make saucepan handles because they are bad
conductors of heat, and also, on heating, these plastics do not get softened.

b) Bakelite is a kind of thermosetting plastic which is a bad conductor of heat and

electricity. Because of this property, it is used for making electric plugs, switches, plug
boards, etc.

8. Categorise the materials of the following products into ‘can be recycled’ and
‘cannot be recycled’.

Telephone instruments, plastic toys, cooker handles, carry bags, ballpoint pens,
plastic bowls, plastic covering on electrical wires, plastic chairs, electrical switches.

Soln:
 Can be Recycled Cannot be recycled

Plastic chairs Cooker handles

Plastic bowls Electrical switches

Plastic toys Telephone instruments

The plastic covering on electrical wires

Carry bags

Ballpoint pens

9. Rana wants to buy shirts for summer. Should he buy cotton shirts or shirts made
from synthetic material? Advise Rana, giving your reason.

Soln: Rana should buy a cotton shirt for the summer and not a synthetic shirt. Cotton is
a good absorber of water. So it can soak the sweat coming out of the body and expose
it to the environment. Thus, it helps in evaporating sweat and helps in cooling the body.

10. Give examples to show that plastics are non-corrosive in nature.

Soln: Plastics are non-corrosive in nature; they do not react even if they come in
contact with strong chemicals. For example, the cleaning chemicals that we use at
home are stored in plastic bottles instead of metal containers.

11. Should the handle and bristles of a toothbrush be made of the same material?
Explain your answer

Soln: The handle and bristle of a toothbrush should not be made of the same material,
as the handle of the toothbrush should be hard and strong while the bristle should be
soft and flexible.

12. ‘Avoid plastics as far as possible’. Comment on this advice.

Soln: We should avoid plastics as far as possible because plastics are non-
biodegradable in nature. Once introduced into the environment, they cause pollution. If
burnt, they release poisonous gases, and the plastic bags thrown in the garbage dump
are swallowed by animals, choking their respiratory system and causing fatalities in
them.
13. Match the terms of column A correctly with the phrases given in column B

Soln:

A B

(i) Polyester (d) Fabrics do not wrinkle easily

(ii) Teflon (c) Used to make non-stick cookware

(iii) Rayon (a) Prepared by using wood pulp

(iv) Nylon (b) Used for making parachutes and stockings

14. ‘Manufacturing of synthetic fibres is actually helping conservation of forests’.

Comment.

Soln: The manufacturing of synthetic fibres is helpful in the conservation of forests

because if we use natural fibres, the raw materials for them have to be derived from
plants, which requires cutting off lots of trees. This proves to be helpful in the
conservation of forests.

15. Describe an activity to show that thermoplastic is a poor conductor of electricity

Soln: In order to show that thermoplastic is a poor conductor of electricity, we will

design a circuit. For that, we need a bulb, some wires, a battery, a piece of metal and a
plastic pipe (as shown in the figure below). After setting the experiment, switch on the
current, and you will observe that the bulb glows in the former case. In the latter case,
the bulb does not glow. Hence, a plastic pipe (which is a thermoplastic) is shown to be
a poor conductor of electricity.
Chapter 11 Force and Pressure
Exercise Questions

1. Give two examples each of situations in which you push or pull to change the state
of motion of objects.

Soln: Examples of situations in which you push or pull to change the state of motion of
objects are

1. Pull situations

a) In order to open a drawer, we have to pull it. This action changes the state of motion
of the drawer.

b) To draw water from the well, the rope is pulled. This action changes the state of
motion of the bucket.

2. Push Situations

a) A football is pushed by the foot of a player. This action changes the state of motion
of the ball.

b) In order to change the place of the heavy box from one room to another, we have to
push it. This action changes the motion of the box.

2. Give two examples of situations in which applied force causes a change in the
shape of an object.

Soln: The forces which change the shape of an object are as follows:

i) By pressing the clay between the hands, it deforms.

ii) The shape of the plastic bottle changes by squeezing it.

3. Fill in the blanks in the following statements.

(a) To draw water from a well, we have to __________ at the rope.

(b) A charged body __________ an uncharged body towards it.

(c) To move a loaded trolley, we have to __________ it.

(d) The north pole of a magnet __________the north pole of another magnet.

Soln:

1. To draw water from a well, we have to pull at the rope.

2. A charged body attracts an uncharged body towards it.
 3. To move a loaded trolley, we have to pull or push it.
4. The north pole of a magnet repels the north pole of another magnet.

4. An archer stretches her bow while taking aim at the target. She then releases the
arrow, which begins to move towards the target. Based on this information, fill up the
gaps in the following statements using the following terms.

muscular, contact, non-contact, gravity, friction, shape, attraction

(a) To stretch the bow, the archer applies a force that causes a change in its
__________.

(b) The force applied by the archer to stretch the bow is an example of
__________ force.

(c) The type of force responsible for a change in the state of motion of the arrow is an
example of a __________ force.

(d) While the arrow moves towards its target, the forces acting on it are due to
__________ and that due to __________ of air

Soln:

(a) To stretch the bow, the archer applies a force that causes a change in its shape.
(b) The force applied by the archer to stretch the bow is an example of muscular force.
(c) The type of force responsible for a change in the state of motion of the arrow is an
example of a contact force.
(d) While the arrow moves towards its target, the forces acting on it are due
to gravity and that due to friction of air.
5. In the following situations, identify the agent exerting the force and the object on
which it acts. State the effect of the force in each case.

(a) Squeezing a piece of lemon between the fingers to extract its juice.
(b) Taking out paste from a toothpaste tube.
(c) A load suspended from a spring while its other end is on a hook fixed to a wall.
(d) An athlete making a high jump to clear the bar at a certain height
Soln: a) We make a muscular force to extract the juice of the lemon by squeezing it. As
a result, the shape of the lemon gets changed.

b) To take out paste from the toothpaste tube, we use our muscular force. This
muscular force acting on the toothpaste tube leads to a change in its shape.

c) Here, the suspended load exerts a force on the spring and pushes the spring
downwards. As a result, the spring gets stretched. Hence, its shape gets changed.

d) The feet of the athlete exert muscular force on the ground, which pushes the ground.
This allows them to jump over the bar. As a result, their state of motion gets changed.
6. A blacksmith hammers a hot piece of iron while making a tool. How does the force
due to hammering affect the piece of iron?

Soln: A blacksmith uses their muscular force while hammering a hot piece of iron. The
muscular force changes the shape of the iron so that it can be given the desired
shape.

7. An inflated balloon was pressed against a wall after it had been rubbed with a
piece of synthetic cloth. It was found that the balloon stuck to the wall. What force
might be responsible for the attraction between the balloon and the wall?

Soln: When an inflated balloon is rubbed with a piece of synthetic cloth, it becomes
charged. A charged body attracts an uncharged body. When this charged balloon is
pressed against a wall, it sticks to the wall. Thus, the electrostatic force acts between
the charged balloon and the wall.

8. Name the forces acting on a plastic bucket containing water held above ground
level in your hand. Discuss why the forces acting on the bucket do not bring a change
in its state of motion.

Soln: For holding the bucket of water above the ground, we use muscular force. This
muscular force acts against the force of gravity that pulls the bucket towards the
ground. The two forces are equal in magnitude but opposite in direction. Therefore, the
net force on the bucket is zero. Hence, there is no change in the state of motion.

9. A rocket has been fired upwards to launch a satellite in its orbit. Name the two
forces acting on the rocket immediately after leaving the launching pad.

Soln: The force of gravity is the one which acts on the rocket to pull it towards the
ground, and the other one is the force of friction due to the earth’s atmosphere, which
opposes its motion.

10. When we press the bulb of a dropper with its nozzle kept in water, the air in the
dropper is seen to escape in the form of bubbles. Once we release the pressure on the
bulb, water gets filled in the dropper. The rise of water in the dropper is due to

(a) pressure of water

(b) gravity of the earth
(c) shape of rubber bulb
(d) atmospheric pressure
Soln: a) Due to the atmospheric pressure, there is a rise of water in the dropper.

When all the air escapes from the nozzle, the atmospheric pressure, which is acting on
the water, forces the water to fill up the nozzle of the dropper.